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Cosentino, Antonino Practical notes on technical for art examination Conservar Património, núm. 21, junio, 2015, pp. 53-62 Associação Profissional de Conservadores Restauradores de Portugal Lisboa, Portugal

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Practical notes on ultraviolet technical photography for art examination

Antonino Cosentino

Cultural Heritage Science Open Source, chsopensource.org. Piazza Cantarella 11, Aci Sant’Antonio, 95025, Italy [email protected]

Abstract Keywords This paper discusses two technical photographic methods based on the use of ultraviolet sources Ultraviolet for art examination: ultraviolet fluorescence (UVF) and ultraviolet reflectance (UVR). While the photography UVF technique can be carried out with any digital , UVR are acquired with a digital Ultraviolet reflectance camera modified to be full spectrum, and thus sensitive into the UV region of the spectrum until photography about 360 nm. This modification involves the removal of the in-built hot mirror filter on the CCD or Technical photography CMOS sensor, rendering it sensitive to ultraviolet and radiation. This paper illustrates the procedures and the equipment needed for these technical photographic methods, discussing case studies and results on prepared samples.

Notas práticas sobre a fotografia de ultravioleta para o exame de obras de arte

Resumo Palavras-chave Este artigo discute dois métodos fotográficos baseados no uso da radiação ultravioleta para o Fotografia de fluorescência exame de obras de arte: a fluorescência de ultravioleta (UVF) e a reflectância de ultravioleta (UVR). de ultravioleta Enquanto a fotografia de UVF pode ser efectuada com qualquer máquina fotográfica digital, as Fotografia de reflectância imagens de UVR são adquiridas com uma máquina fotográfica digital modificada para espectro de ultravioleta completo, sensível à radiação ultravioleta com comprimento de onda até cerca de 360 nm. Esta Fotografia técnica modificação envolve a remoção do filtro interno sobre o sensor CCD ou CMOS de forma a tornar a máquina sensível à radiação ultravioleta e infravermelha. Este artigo descreve os procedimentos e o equipamento necessários para a realização desses exames, discutindo quer estudos de casos quer os resultados obtidos com amostras preparadas com esse objectivo.

ISSN 2182-9942

Conservar Património 21 (2015) 53-62 | doi:10.14568/cp2015006 ARP - Associação Profissional de Conservadores-Restauradores de Portugal http://revista.arp.org.pt Antonino Cosentino

Introduction artwork; and to identify retouches on polychrome art. It must be noted that UVF is also known as UV induced Technical Photography (TP) for art diagnostics visible fluorescence. Both terms are descriptive of the corresponds to the collection of broadband spectral phenomenon and the second is simply more detailed. images acquired with a modified full spectrum digital This paper illustrates recent applications together with camera, using different illumination sources and filters. recommendations for the photographic equipment and In addition to capturing visible , the silicon-based procedures that take into account the latest technological CCD or CMOS respond both to the near- developments. However, this paper does not address the infrared and near-ultraviolet ranges of the spectrum interpretation of the actual fluorescence of art materials. (between about 360 and 1100 nm). Manufacturers install Other studies have discussed UV fluorescence spectra of an UV/IR cut-off filter in front of the sensor to reduce painting material (pigments, dyes, media and varnishes) the acquisition of transmitted infrared and ultraviolet [9-11] and provide useful resources to conservators and radiation. Specialized companies can remove this filter conservation scientists in their interpretation of the UVF and then the camera is said to be modified full spectrum. images. Nevertheless, it must be taken into account that The full-spectrum camera may then be used successfully the spectral emission of commonly fluorescent artists’ to acquire broadband images in the infrared and materials can be highly modified by the presence of other ultraviolet regions, which can reveal information such surrounding materials. Optical emitted radiation can as underdrawing, pentimenti and retouches in paintings interact with surrounding matter, giving rise to selective [1-4]; enhance the reading of faded wall paintings [5]; absorption and scattering phenomena [12]. or be used for archaeological documentation [6]. These technical photos can also provide preliminary spectral information regarding the identification of materials [7] Experimental based on their characteristic response in the different radiation wavebands. Even if the conclusions that can be UV radiation sources drawn from these images are qualitative and not absolutely conclusive, this methodology is getting a lot of interest in For the documentation of paintings, a lamp power the art conservation field because it is simple and relatively output of around 50 µW/cm2 is recommended so that the affordable, thus making it an ideal first step for pigment shooting is limited to a few seconds and there are identification, before other techniques are carried out. no damages to the artworks [13]. This radiation is much This paper discusses technical photography methods less than the natural UVA radiation from the sun which that use UV radiation sources: UV fluorescence (UVF) is in the range 500-1500 µW/cm2 [8]. The typical practice and UV reflectance (UVR). As long as the UV dosage for conservation studios was to use UV fluorescent is kept within a reasonable limit [8], these methods can tubes. They are very cheap and can be mounted on be considered safe and non-invasive and can be used to commercial fixtures for normal visible light fluorescent characterize painting materials; to evaluate the current tubes, provided that the support is not UV fluorescent state of conservation and prior treatment history of an itself. These tubes are very economical, and as many as

Figure 1. UV LED lamps must be filtered with a UV-pass filter to cut off the purple visible emission, which would otherwise limit their utility for UV technical photography. Emittance spectra show the violet component of the LED is cut off. A fingerprint on the wall becomes visible only when the UV-pass filter is applied on the lamp.

54 Conservar Património 21 (2015) Practical notes on ultraviolet technical photography for art examination

Figure 2. In the late ‘50s, postage stamps from countries such as the UK, France and the USA were coated with in order to automate the postal sorting and cancelling. There are two Ultraviolet wavelengths used for the examination of stamps. Long-wave (UVF at 365 nm) is used to detect alterations, damages and repairs. Phosphors are best viewed in shortwave ultraviolet radiation (UVF254). While the Canadian stamp (upper row) shows the yellow fluorescence of its with both UVF and UVF254, the US and French stamps show orange and green phosphors, respectively, only in the UVF254 photo. are needed can be used to provide the UV illumination the same UV radiation quality of the forensic lamps but necessary for studio photography. The important with more advantages. They turn on instantly, do not heat drawback is that since the surface of the tubes is very large up, and they are very lightweight and sturdy. These LEDs they cannot be practically filtered in order to cut off the must be filtered to cut out the violet light output with a visible violet light and the infrared radiation that is also UV-pass glass analogous to the BW 403 filter (Figure 1). produced. This has been a condition accepted by most On the other hand, there are also UV LEDs available from conservation studios, especially since the examination other brands (e.g. Roithner), which output a very narrow was limited to detect inpaints (which generally will look UV band and no violet contamination. darker than old paints and varnishes). On the other hand, The UV lamps mentioned so far have a main emission the violet light will not adequately allow for the subtle peak at 365 nm. UV fluorescence can also be induced UV fluorescence from old paints and varnishes to with lamps emitting at 254 nm, and this method is called be fully appreciated. An alternative to the tube lamps is UVF254. These lamps must be used with extreme care a forensic UV spotlight lamp, mounting a mercury vapor since this UV range (UVC) is dangerous for the eyes lamp (in the range of 150 W) with internal ballast and a and skin. Illumination with a 254 nm UV lamp produces UV pass filter, providing excellent filtration. Indeed, since fluorescence in molecules not excited by the 365 nm UV this is a spotlight, illumination is not uniform over the lamp (Figure 2), and this technique allows these events to painting area, so a “light painting” technique is necessary. be observed. To solve this, the lamp is shone over the painting during a long exposure, in order to have a final UV that has Camera and photo calibration been uniformly exposed. It is often necessary to document works of art of large For VIS (visible) and UV fluorescence photography dimensions with ultraviolet photography, either UVF and/ the camera is calibrated with the X-rite ColorChecker or UVR. In this case, the panoramic photographic method Passport, the industry standard reference target [14, 15] can be implemented. for creating DNG profiles for so However, the UV lamps often provide uneven and specific colors may be evaluated. Due to differences in insufficient illumination for large surfaces. In this case, technologies and variables in manufacturing processes the panoramic method must be coupled with the light every camera captures colors a bit differently and must painting technique, using a spotlight UV lamp along with undergo custom calibration. The images are shot in the panoramic head. camera RAW format and they are then color corrected For UV photography (UVF and UVR) the high flux using the DNG camera profile mentioned above. Nichia 365 nm LED lamps are recommended. They have The American Institute of Conservation Photo Conservar Património 21 (2015) 55 Antonino Cosentino

red fluorescence of the red patch: Red channel 70 ± 5, Green 0, Blue 0. The AIC PhD target is also used to exposure correct the UVR image. Its grey patches are identified by the following designations (white to black): white; N8; N6.5; N5; N3.5; and black. The N8 patch is used to exposure correct the UVR image (RGB 50 ± 5). Filters

For the two ultraviolet photography methods illustrated in this paper a set of three filters was used, and Figure 3 shows their transmittance curves. Other than the proposed filter set, many other possible combinations currently exist to obtain similar results. This set was selected based on an optimization to select better quality filters relative to their cost. Figure 3. Transmittance curves of the three filters used for UVF and UVR photography. Figure 4 shows the experimental setup to perform UVF and UVR photography, indicating the position of the camera and the lamps, and the filters used. UVF Documentation (AIC PhD) target [16] is also captured photography uses two filters – the X-Nite CC1 coupled along with the work of art and it is used for white with the BW 420. The CC1 is necessary to cut off any balancing VIS photography. Color calibration for infrared radiation produced by the UV lamp. X-Nite ultraviolet fluorescence (UVF and UVF254) is trickier as CC1 belongs to the hot mirror class of filters, including there is not yet an officially recognized reference standard. the BW 486 and Heliopan BG38. They are used to stop As an aid for color balancing, and in order to guarantee the infrared but usually allow some UV. Therefore they reproducibility of the UVF photos, the AIC PhD target cannot be used alone for UVF photography; otherwise is accompanied by three emitters activated by UV the images acquire a purple cast. Indeed, a full spectrum lamps: a section of a card for forensic UV photography camera becomes sensitive to UV radiation, and therefore (orange fluorescence), a swatch of zinc white (yellow the reflected UV radiation must be shielded from the fluorescence), and a fluorescent paint (green fluorescence). sensor, otherwise the images will be dominated by the These three UV emitters, together with the red purple color associated to the UV radiation that is picked fluorescence emission of the red swatch of the AIC up by the red and blue channels. The BW 420 is a UV PhD target, are used for color calibration of UVF and cut-off filter necessary to stop the UV reflected by the UVF254 photos. This is performed on the RAW files by subject and passed by the X-Nite CC1. If it is necessary editing them with Adobe Camera RAW software and to make close up UVF photography, the BW 420 is not assigning the appropriate Temperature and Tint. The UV suitable because it also fluoresces under UV radiation. fluorescence images are also exposure corrected using the It is therefore necessary to substitute it with the UV/

Figure 4. Experimental setup for the two ultraviolet photographic methods, UVF and UVR. 56 Conservar Património 21 (2015) Practical notes on ultraviolet technical photography for art examination

Figure 5. Pigments checker, collection of 54 historical pigments swatches. In the top row, images were taken with a 200 mm (f/8) lens at a 2 m distance. A 50 mm (f/8) lens at a 0.7 m distance was used for the bottom row. The lamp and the subject remained in the same relative position and only the camera was moved forward. UVF CC1 are the images taken with only the X-Nite CC1 filter and they are purple due to the UV radiation having been reflected into the camera and detected by the CMOS sensor. UVF CC1 images are also out of focus because the reflected UV dominates the images, and the lens was focused on the VIS instead. Once the BW420 or the UV/IR Baader filter is added, it becomes possible to the actual UV fluorescence by canceling the noise brought by the reflected UV radiation. The images at 0.7 m are slightly affected by the yellow fluorescence of the BW 420 filter, as shown in the detail of the zinc white swatch (far right).

IR Baader filter [17] which has the same transmittance photographic lens can be used without any issues. It is curve but does not emit any fluorescence. If the picture recommended to use fixed focal lenses and avoid complex is taken from a distance and the UV lamps are kept at a lenses, as they are likely to give out flares in the ultraviolet close angle, the reflected UV that hits the BW 420 filter reflectance photography. It is also recommended to avoid is only a small fraction of the total UV fluorescence, telephoto lenses over 200 mm, so that the lens will be fast and so the fluorescence produced by the filter does not enough to work with the low-intensity emission produced affect the final image. On the other hand, if a close up in UV fluorescence imaging. It must be noted that after UVF photo is taken, the amount of UV hitting the filter shooting a VIS photo, it is necessary to refocus for UVR could have considerable adverse effects on image quality photography since UV radiation is focused on a different (Figures 5-6). The absorbing filters, such as BW 420 and focal plane than that of visible light. Wratten gelatins, typically show a fluorescence emission induced by the absorbed UV radiation. On the other hand, reflective filters which exploit radiation interference by multi-layer coatings typically do not suffer from these problems, although they are more expensive. For UVR, a UV-pass filter, the BW 403, is coupled to the X-Nite CC1 to stop possible infrared radiation from the lamp or the environment. This filter does not produce any fluorescence and can be used on close-up photography. Lenses

For UV reflectance photography old lenses often perform better, since the new ones can have optical acrylic elements and “anti-reflective” lens coatings which can significantly cut UV radiation. It must be mentioned that specialized and costly UV lenses exist and are mainly used for applications in forensics, science, and fine art photography. These are made of quartz or calcium fluoride and effectively transmit and focus UV radiation. On the other hand, there is no special lens requirement Figure 6. The UV induced fluorescence of the BW420 filter becomes apparent when the filter is photographed under UV for UVF photography, since this technique involves radiation. For close-up photos, the UV/IR Baader filter is strictly capturing the visible region of the spectrum, and a normal recommended. Conservar Património 21 (2015) 57 Antonino Cosentino

Information provided by the the actual fluorescence of the pigments. Consequently, and its interpretation UVF documentation aiming at pigments identification is recommended only when the varnish has been removed Ultraviolet fluorescence from the artwork. UVF photography must always be seen as a supplementary technique for pigment identification, UV fluorescence describes the emission of visible since this method can only provide qualitative information light observed when an electron of a molecule or atom that on the UV-induced visible fluorescence of materials. For has been excited to a higher energy state by UV radiation quantitative studies, the UV fluorescence must instead relaxes to its ground state, emitting a photon with energy be documented with fluorescence spectroscopy and associated to the difference between the two states. The colorimetric measurement. emission induced on polychrome artworks depends on Specifically in the case of paintings, UV fluorescence many factors: the wavelength and bandwidth of the UV largely comes from the surface of the top paint layer, source, the painting materials (pigments, dyes, binders, and it is barely influenced by the underlying layers. varnishes) and how all of these materials interact with Conservators pursue UVF photography to identify and each other and, additionally, how they have aged. UVF is evaluate the integrity of old varnish layers and to localize used for the examination of different kinds of historical inpaints, which generally lack fluorescence, in contrast and archaeological objects [18], including photographs with the bright appearance of aged varnishes. When the [19] and textiles [20], but it is most widely used for contrast is intense enough, it is possible to implement an polychrome art. UV fluorescence can indeed be used for a automatic segmentation of the UVF image of a retouched preliminary identification of pigments [21-23]. However, painting [24]. This procedure can quicken the workflow the varnish does play a major role, since it generally for the estimation of the extent of losses, a necessary step exhibits a strong fluorescence which can overwhelm in the preparation of condition reports. Regarding wall paintings, the binding material of a true fresco does not add any fluorescence to the pigment, while the secco, or dry, technique is clearly identified by the fluorescence emission of the binding media used to lay the paint. A characteristic of baroque murals was that the pigments were applied as an opaque mass rather than in a thin, almost translucent layer used in the buon fresco method [25]. This technique allowed impastos of varying thickness in a manner analogous to that of oil painting to be rendered for entire murals. A cycle of such 18th century frescoes was revealed in the Crucifix Chapel of the Mother Church in Aci Sant’Antonio (Sicily) in 2012. The paintings have survived along the corners of an originally square chapel that, in the early 20th century, was altered to acquire the current octagonal plan. The paintings have been recently examined with technical photography [26]. The paintings lack giornate, the line between each day’s work, and only pontate, the break between levels of scaffolding, can clearly be seen indicating that a mixture of fresco and secco techniques would have been used. This hypothesis is corroborated by the intense fluorescence of the paint under UV radiation, suggesting the presence of an organic binder (Figure 7). Ultraviolet reflectance

As for UVF, since UV radiation interacts mostly just with the superficial layer, the UVR image is specific to the topmost pigments or varnish. UVR is recognized as a useful tool to identify white pigments like titanium white and zinc white, which show a strong UV absorbance band [27] and are thus identifiable for appearing very dark in the UVR image, especially when compared to other white pigments, namely lead white and lithopone, which appear bright (Figures 8-9). A UV false color (UVFC) method Figure 7. Crucifix chapel, Aci Sant’Antonio (Sicily). Thea secco technique is suggested by the intense UV fluorescence. has been proposed [28] which, in a manner analogous 58 Conservar Património 21 (2015) Practical notes on ultraviolet technical photography for art examination

Figure 8. UVF photo shows a characteristic yellow fluorescence, suggesting zinc white for the white pigment used for the background. The UVR image also validates this assignation since the pigment absorbs the UV and appears dark. On the other hand, the restoration interventions on the cracks appear as dark spots in the UVF image, as is normal with recent inpaints. They also are identified in the UVR image, since the inpaints reflect UV. They likely include lithopone, as suggested by the dull UV fluorescence. UVR (Luminosity) (opacity 70%) allows a more intuitive reading of the location of the inpaints. Artwork: 20th century oil on canvas, private collection.

Figure 9. This painting has been heavily restored. Due to more contrast with the original pigments, the retouches can be better identified in the UVR image than in the UVF. UVR (Luminosity) (opacity 70%) allow the retouches to be read in the context of the actual colours. 20th century oil on canvas, private collection. to the infrared false color, mixes UV reflectance and UVR (Luminosity), allows a more intuitive reading of VIS channels. Here, it is instead recommended to the information provided by the UV reflectance photo combine the VIS and UVR images with the luminosity (Figures 8-9). blending mode available in photo editing software such Reflectance photography in the shorter UV range as Adobe Photoshop. This mode keeps the luminance of does not seem to provide much more useful information. the upper layer (UVR), and blends it with the hue and Figure 10 shows the UV reflectance imaging of a saturation of the lower layer (VIS). The resulting image, pigments checker with 54 historical pigment swatches Conservar Património 21 (2015) 59 Antonino Cosentino

Figure 10. Spectral images in the UV region acquired with SpectroCam UV-VIS and filters in the UV region.

Figure 11. Lithopone has an absorbance band in the shorter UV region, which is measured in the reflectance spectra and can be visualized in the spectral image at 330 nm. 60 Conservar Património 21 (2015) Practical notes on ultraviolet technical photography for art examination taken with a PixelTeq UV-VIS camera whose sensitivity Science 6(1) (2015) 23-34, http://www.ijcs.uaic.ro/public/ is extended to the UV region until about 200 nm. Three IJCS-15-03_Cosentino.pdf. spectral images were taken using different bandpass filters 6 Verhoeven, G., ‘Imaging the invisible using modified digital still cameras for straightforward and low-cost (center wavelength / bandwidth): 330/25, 365/19, 380/12. archaeological near-’, Journal of The pigments checker was illuminated with a UV lamp, Archaeological Science 35 (2008) 3087-3100, doi:10.1016/j. model B100 AP manufactured by UVP (100 W spot jas.2008.06.012. mercury bulb). The only useful information that resulted 7 Cosentino, A. ‘Identification of pigments by multispectral imaging; a flowchart method’,Heritage Science 2:8 (2014) from this test was the observation of the absorbance band doi:10.1186/2050-7445-2-8. of lithopone in the far UV region (Figure 11). 8 Warda, J.; Frey, F.; Heller, D.; Kushel, D.; Vitale, T.; Weaver, G., ‘Ultraviolet photography’, in AIC Guide to and Conservation Documentation, 2nd Conclusions ed., American Institute for Conservation of Historic and Artistic Works, Washington DC (2011). 9 Pelagotti, A.; Pezzati, L.; Bevilacqua, N.; Vascotto, V.; This paper has illustrated technical solutions for Reillon, V.; Daffara, C., ‘A study of UV fluorescence two photographic methods that use UV radiation: UV emission of painting materials’, in 8th International Conference on Non-Destructive Testing and Microanalysis fluorescence and UV reflectance. The necessary filters, for the Diagnostics and Conservation of the Cultural and lenses, lamps, set up, camera and photo calibration have Environmental Heritage, Lecce, Italy (2005), http://www. been discussed. Applications have also been reported in inoa.it/home/lella/pdf/Art2005_A%20study%20of%20 order to show the potential of a modified full spectrum UV%20fluorescence%20emission%20of%20painting%20 ma%85.pdf. camera to perform these technical photographic methods 10 Pelagotti, A.; Pezzati, L.; Piva, A.; Del Mastio, A., when equipped with the proper accessories. In addition, ‘Multispectral UV fluorescence analysis of painted surfaces’ the luminosity blending mode has been introduced for in Proceedings of 14th European signal processing the first time to make the reading of UVR images more conference (EUSIPCO), Firenze, Italy (2006). intuitive. It is expected that due to the relatively low cost of 11 Verri, G.; Comelli, D.; Cather, S.; Saunders, D.; Piqué, F., the technical photographic equipment, this methodology ‘Post-capture data analysis as an aid to the interpretation of ultraviolet-induced fluorescence images’, inComputer will be readily accepted for use across the conservation Image Analysis in the Study of Art, ed. D. G. Stork and sector. Further research and results are expected from J. Coddington, SPIE, San Jose (2008) 681001-681012, UV-fluorescence multispectral imaging which has proven doi:10.1117/12.764463. already [29] to be a promising tool for polychrome art 12 Verri, G.; Clementi, C.; Comelli, D.; Cather, S.; Piqueé F. ‘Correction of ultraviolet-induced fluorescence spectra examination. for the examination of polychromy’, Applied Spectroscopy 62(12) (2008) 1295-1302, doi:10.1366/0003702087868222 96. Acknowledgments 13 Feller, R. L., Accelerated Aging. 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21 Isacco, E.; Darrah, J., ‘The ultraviolet-infrared method 28 Aldrovandi, A.; Buzzegoli, E.; Keller, A.; Kunzelman, D., of analysis, a scientific approach to the study of Indian ‘Investigation of painted surfaces with a reflected UV false miniatures’, Artibus Asiae 53(3-4) (1993) 470–491, color technique’, in Art’05 — 8th International Conference doi:10.2307/3250528. on “Non Destructive Investigations and Micronalysis for the Diagnostics and Conservation of the Cultural and 22 René de la Rie, E., ‘Fluorescence of Paint and Varnish Environmental Heritage, Lecce (2005). Layers (Part I)’, Studies in Conservation 27(1) (1982) 1-7, doi:10.1179/sic.1982.27.1.1. 29 Comelli, D.; Valentini, G.; Nevin, A.; Farina, A.; Toniolo, 23 Comelli, D.; Nevin, A.; Brambilla, A.; Osticioli, I.; L.; Cubeddu, R., ‘A portable UV-fluorescence multispectral imaging system for the analysis of painted surfaces’, Valentini, G.; Toniolo, L.; Cubeddu, R., ‘On the discovery Review of Scientific Instruments of an unusual luminescent pigment in Van Gogh’s painting 79(8) (2008) 086112, doi: “Les Bretonnes et le Pardon de Pont Aven”’, Applied Physics 10.1063/1.2969257. A, 106(1) (2012) 25-34, doi:10.1007/s00339-011-6665-9. 24 Cosentino, A.; Stout, S., ‘Photoshop and multispectral Received:: 29 May 2015 imaging for art documentation’, e-Preservation Science Revised: 5 July 2015 11 (2014) 91–98, http://www.morana-rtd.com/e- Accepted: 13 July 2015 preservationscience/2014/ePS_2014_a11_Cosentino.pdf. Online: 24 July 2015 25 Mora, P.; Mora, L.; Philippot, P, Conservation of Wall Paintings, Butterworths, London (1984). 26 Cosentino, A.; Stout, S.; Di Mauro, R.; Perondi, C., ‘The Crucifix Chapel of Aci Sant’Antonio: newly discov­ered frescoes’, Archeomatica 2 (2014) 36–42, http://issuu.com/ This work is licensed under the Creative Commons geomedia/docs/archeomatica_2_2014?e=1225360/9272033. Attribution-NonCommercial-NoDerivs 3.0 Unported License. 27 Cosentino, A., ‘FORS spectral database of historical To view a copy of this license, visit pigments in different binders’,e-conservation Journal, 2, http://creativecommons.org/licenses/by-nc-nd/3.0/deed.pt. (2014) 57-68, http://e-conservation.org/issue-2/36-FORS- spectral-database.

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